Electromagnetic heating has been used in the past for treating tumors. However, when the tumors are located deep within the body of the patient, it has been found to be very difficult to deliver sufficient hyperthermic energy to the tumorous tissue to destroy the tumorous tissue without adversely affecting surrounding healthy tissue or even healthy tissue in the path between the power source and the tumorous tissue. To avoid damage to healthy tissue, it is necessary to focus the microwave energy on the tumorous tissue. It has been found that it is very difficult to focus the microwave energy.
One solution has been to use phased-array antennas. Theoretically phased-array antennas can be used to actually focus the microwave energy for the heat treatment of the tumor without adversely affecting healthy tissue. The problem with the use of phased-array antennas is that it is necessary to determine the relative phase of excitation in each individual element of the array so that the electromagnetic energy is properly focused to the prescribed tumorous region. If the phased-array antennas were acting through a homogeneous medium then the phase determination could be accomplished by geometric computations. However, since the actual biological media through which the microwaves travel is far from homogeneous geometric computations are not practical. Such computations require the knowledge, along other things, of the exact tissue homogeneity distribution in the path of each ray from each array element. Accordingly, the solution has in the past been evasive.
The proposed solutions to the tissue inhomogeneity problems have included tissue temperature measurements on the tumor and surrounding tissue and conjugative methods. The temperature measuring methods include planting invasive temperature probes and using a feedback algorithm for varying the amplitude and/or phase outputs of the various array elements to maintain the tumor above a given temperature and the surrounding tissue below a given therapeutic temperature (42-44 degrees C. for example). See for example the articles entitled "Optical Temperature Control with Phased-Array Hyperthermia System" and "A Predictive Adaptive, Multipoint Feedback Controller for Local Heat Therapy of Solid Tumors" both published in the IEEE Transactions on Microwave Theory and Techniques", Vol. MTT-34, No. 5, May 1986.
Another solution proposed is the use of a probe invasively inserted into the patient juxtaposed to the tumor. See the article entitled: "Experimental Investigation of a Retro-focusing Microwave Hyperthermic Applicator: Conjugate Field Matching Scheme" written by J. Loane et al., and other articles published in the IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34 No. 5 May (1986). The invasive probe is used to transmit radio frequency or microwave energy. The phased-array antenna is used in this mode as a receiving antenna. The signals received by the phased-array antenna are analyzed to determine the phase and the amplitude of the received signal at each element relative to each of the other elements of the array. When focusing the microwave or radio frequency energy, the array elements are excited with energy that is proportional to the conjugate of the phase of the individual array elements in the receiving mode. By reciprocity the phase and amplitude of the radiated field from the array is thereby focused exactly at the tumor. Theoretically the method works regardless of the inhomogeneity of the medium through which the radio waves pass or differences between the array elements since the element differences and the inhomogeneity are cancelled out by the conjugate and reciprocity theorems. In both proposed solutions, however, hyperthermia treatment requires invasive means.
Accordingly, those skilled in the art are continuously searching for a means for focusing the radio frequency energy or microwave energy on to the tumorous tissue in a non-invasive manner. In addition, it is often not feasible to put a transmitter juxtaposed to the tumorous tissue. For example, when the tumorous tissue is in a critical area of a vital organ. Accordingly, those skilled in the art are seeking non-invasive methods of focusing the microwave or radio frequency energy used in hypothermic treatment of cancerous or tumorous tissues.